The present invention concerns an automatic transmission used in a vehicle, and more particularly the arrangement of planetary gear units in connection with friction elements worked by hydraulic pressure.
Generally the automatic transmission for a vehicle includes a transmission control unit, which automatically controls a plurality of clutches and brakes mounted in the gear train so as to work the three fundamental elements (sun gear, ring gear, planetary carrier) of the planetary gear units as the torque input or output elements, thus adjusting the final output of the transmission.
The power train of the automatic transmission must normally include a composite planetary gear device and at least five friction elements in order to produce four forward speeds and one backward speed. However to improve the shifting operation more effectively a composite planetary gear unit, seven friction elements and three one-way clutches are required. This causes a complicated construction of the power train, and increase of the weight resulting in the degradation of the fuel consumption rate.
Moreover, most of the automatic transmissions involving planetary gear units generally have the input shaft on which are mounted all of the planetary gear units and friction elements mounted on the input shaft, so that the number of parts for connecting the planetary gear units with the friction elements is increased as are length and weight of the transmission. Additionally, the lines for supplying hydraulic pressure to the friction elements are complicated as well as the lubrication lines.